1. Field of the Invention
The invention relates to a steel particularly useful for cylinder ferrules for continuous casting of aluminum having, with respect to the steels currently known, an improved life span without any deterioration in the productivity of the installation.
2. Discussion of the Background
The continuous-casting machines for aluminum, or aluminum alloys, consist of two rotating cylinders between which the liquid metal is introduced. In certain recent installations, these two cylinders rest on two larger support cylinders. The cylinders in contact with the liquid metal are designed to solidify the latter and form a sheet metal which in addition undergoes or may undergo a hot-rolling making it possible to obtain it in the form of a coil. For this purpose, the cylinders are made up of a central portion (core) bearing cooling circuits and a ferrule mounted on the core by binding or any other means.
The inner surface of the ferrule is cooled by the water circulating in cooling tubes borne by the core. The primary role of the ferrule thus is to extract the calories from the liquid aluminum to make possible the solidification thereof prior to release from control between the cylinders.
The ferrule, cooled internally, thus constitutes a heat exchanger with the liquid aluminum. The productivity of the installation depends directly on the power of this exchanger.
The ferrule furthermore is subjected to intense thermomechanical actions due to thermal cycling and to constraints of mechanical origin: mounting constraints, in particular from binding resulting from the design of the cylinders--bending and torsion constraints due to operating stress. These actions lead to a surface plastic fatigue, the initiation and the spreading of networks of microfissures, which necessitate a periodic reconditioning of the ferrule, by machining. The life span of a ferrule thus depends essentially on its capacity to withstand thermal actions.
A steel for a continuous-casting ferrule therefore should possess both:
good heat-exchange capacity with liquid aluminum in order to ensure high productivity of the installation, and
good resistance to thermal fatigue in order to provide a long life span for the ferrules.
Up to the present:
the heat exchange capacity has been linked essentially to the thermal conductivity of the variation at ambient temperature,
the resistance to thermal fatigue has been linked to the mechanical and physical characteristics at ambient temperature or when heated (Limit of elasticity, Young Model, coefficient of dilatation . . . ) or, more precisely, has been measured in simulation tests.
Generally speaking, the addition of alloy elements Cr, Mo, V, improves the capacity to withstand thermal fatigue but degrades thermal conductivity at ambient temperature.
The search for variations suitable for the manufacture of ferrules thus has been confined, up to the present, to that for a conductivity/thermal fatigue compromise by optimal adjustment of the ratios among the various alloy elements and by restriction to relatively scantly alloyed variations.
It is thus that the steels disclosed in the patent U.S. Pat. No. 4,409,027 (incorporated herein by reference) are known, containing in their composition:
0.5 to 0.6% carbon
0.4 to 1% manganese
0.1 to 0.3% silicon
0.4 to 0.9% nickel
1.5 to 3% chromium
0.8 to 1.2% molybdenum
0.3 to 0.5% vanadium,
and those in the patent FR 2,567,910 (incorporated herein by reference) containing:
0.30 to 0.65% carbon
Max. 0.80% manganese
Max. 0.80% silicon
2 to 4.5% chromium
0.4 to 0.8% molybdenum
0.1 to 0.3% vanadium
The patent U.S. Pat. No. 4,861,549 (incorporated herein by reference) recommends additions of rare earths in the basic compositions, as for example:
0.45 to 0.49% carbon
0.90 to 1.00% manganese
0.15 to 0.35% silicon
1.2 to 1.5 nickel
1.2 to 1.45% chromium
0.8 to 1.0% molybdenum
0.15 to 0.20% vanadium
0.08% max. rare earths
The best compromise appears to be provided by the variation of the ferrule in the patent application FR 85,03,867 (incorporated herein by reference), the composition of which is the following:
0.30 to 0.36% carbon
0.30 to 0.60% manganese
0.15 to 0.45% silicon
Max. 0.40% nickel
2.80 to 3.40% chromium
0.85 to 1.25% molybdenum
0.10 to 0.30% vanadium
It will be noted that none of these current state-of-the-art compositions combines chromium with another alloy element in a percentage in excess of 1.5%, for fear of a reduction in thermal conductivity.